1. Field of the Invention
The present invention is directed to an apparatus for determining pressure and pressure differentials. More particularly, the invention is directed for use in determining the pressure differentials and fluid density in oil wells. The invention has utility in applications such as high pressure oil well monitoring.
2. Prior Art
In order to monitor and project the production abilities of wells, such as oil wells, it is necessary to gather information about the pressure of the well and the density of the fluid being produced from the well. Information regarding the depth of the oil field, the permeability of the oil producing formation, the head pressure of the well, and the projected oil producing ability and speed of production are necessary to accurately predict and monitor an oil well in order to maximize oil production and profits. Thus, tests are conducted on oil wells to monitor these characteristics and determine the appropriate course of action for maximizing the production of an oil field. The standard methods for measuring the well characteristics and monitoring the production ability of the wells are performed by lowering a pressure monitoring device into the well to monitor characteristics along the depth of the well. The usual method for developing pressure information uses a crystalline detector exposed to the pressure of the well and a second reference crystalline detector which is maintained at a constant pressure. However, these measurements are affected by the temperature variations within the oil well. The known pressure monitoring devices and methods adapt to this temperature change because the reference crystalline detector is maintained in a constant pressure housing which is lowered into the well along with the pressure monitoring crystal so that both crystalline detectors are exposed to the same temperature. Because the housing and encased crystalline reference detector take a different amount of time to react to the temperature changes in the oil well, a waiting period for temperature stabilization is necessary before accurate readings may be taken.
The systems for performing these measurements with crystalline resonators is well known. Quartz resonating systems are described in U.S. Pat. Nos. 3,355,949; 3,561,832; 4,455,875; 4,607,530; 4,802,370; 4,936,147; 5,231,880; 5,299,868 and 5,302,879. Each of these patents is briefly outlined in the following discussion and is hereby incorporated by reference.
U.S. Pat. No. 3,355,949, issued to Elwood et al on Dec. 5, 1967, discloses a crystal temperature and pressure transducer. The specification describes a three crystal temperature and pressure transducer where two crystals are sealed from the effects of pressure and one crystal is exposed to the effects of pressure.
U.S. Pat. No. 3,561,832, issued to Karrer et al on Feb. 9, 1971 discloses a quartz resonator pressure transducer. This specification teaches the construction of a resonator as an intrinsical part of a quartz cylinder. One embodiment of the quartz pressure transducer uses a pair of resonators formed from a single cylindrical core of quartz. In this embodiment, the resonator orientations may be selected to provide maximum, opposite-polarity pressure coefficients of frequency so that the combination of the frequencies will provide a greater pressure sensitivity in a direct beat frequency output without a separate reference crystal. This embodiment also teaches that the temperature-dependent properties of the resonators will be canceled out because both resonators will operate in substantially the same environmental conditions.
U.S. Pat. No. 4,455,875, issued to Guimard et al. on Jun. 26, 1984, discloses a pressure measurement sonde. This invention describes a pressure measurement apparatus which compensates for the effects of the temperature of the surrounding medium. To overcome the drawbacks of temperature variation in the design of the measurement sonde, a heat sink is used along with a heat conductor to equalize the temperature variations between the crystals as rapidly and as effectively as possible in an attempt to balance the heat exchange rate between the two detectors into the surrounding medium.
U.S. Pat. No. 4,607,530, issued to Chow on Aug. 26, 1986, discloses temperature compensation for pressure gauges. The specification describes a method and apparatus for thermally compensating temperature effects on pressure gauges through the use of a fast responding thermal device. The readings of the pressure device are then compensated for by the readings of the thermal device in order to provide a correction to the frequency output of the pressure gauge. This system also utilizes two crystals to measure pressure where one of the crystals is kept at a constant pressure while the other crystal is subjected to an environmental pressure.
U.S. Pat. No. 4,802,370, issued to EerNisse et al. on Feb. 7, 1989 discloses a transducer and sensor apparatus and method. The specification describes an apparatus which includes a pressure sensor, a reference device, and a temperature sensor located within a common environment. For this device, the referenced device and the temperature sensor are constructed to have a temperature response time matched to the temperature response time of the pressure sensor. This allows for compensation for temperature gradient produced by external heating or pressure volume heating. Additional disclosure and claims for this device can be found in U.S. Pat. No. 4,936,147, issued to EerNisse et al. on Jun. 26, 1990 which describes a transducer and sensor apparatus and method.
U.S. Pat. No. 5,231,880, issued to Ward et al on Aug. 3, 1993 discloses a pressure transducer assembly. This specification describes a quartz crystal resonator pressure transducer assembly comprising three crystal resonators. The pressure crystal assembly is immersed in a pressure and temperature transmitting fluid while the temperature and reference crystals are thermally coupled to the fluid but isolated from the pressure by being mounted in a pressure proof enclosure. Electronics are then used to provide a mixed frequency output representative of the pressure and temperature data.
U.S. Pat. No. 5,299,868, issued to Dennis et al on Apr. 5, 1994 discloses a crystalline transducer with an A/C cut temperature crystal. This invention discloses a package that contains a crystal resonator that senses the pressure applied to the housing, along with a crystal resonator that senses the temperature communicated through the housing and a crystal resonator that functions as a reference to compensate for the temperature effects on the pressure sensing resonator. The improvement of this patent discloses a crystalline housing with a crystalline reference resonator mounted in the crystalline housing and a crystalline temperature resonator mounting in the crystalline housing wherein the crystalline temperature resonator is an A/C-cut quartz crystal.
U.S. Pat. No. 5,302,879, issued to Totty et al. on Apr. 12, 1994 discloses a temperature/reference package, and method of using the same for high pressure, high temperature oil or gas well. This specification teaches an improved dual crystal resonator package which includes the first and second crystal embodiment connected together to form an enjoining cavity which includes a crystalline temperature resonator and a crystalline reference resonator for pressure reference. This is then used to generate temperature data signals and reference signals in response to the frequencies of the crystalline temperature resonator and the crystalline reference resonator.
The above described patents and the prior art suffer from the drawbacks of utilizing too many crystals and the problems associated with multiple crystal transducer designs. As shown in FIG. 1, the prior art has a crystalline sensor device exposed to the pressure P1 and a reference crystalline device enclosed in a pressure proof enclosure. The signals of these crystalline devices are combined by a mixer 1 to form an absolute pressure measuring device. In order to do differential pressure monitoring a second sensor crystalline device and a second reference crystalline device must be combined along with mixer 2. Mixer 3 is then used to get the difference between the output of mixer 1 and the output of mixer 2 to find the signal associated with the pressure differential. Thus, this system uses four crystals with each having their own instabilities and jitter, and three mixers which all have their own error patterns. All of this equipment requires a greater amount of air space and a larger package for insertion into the well to be monitored. Further, another crystal must be added to compensate for temperature variations of the devices and this introduces additional errors into the system and requires an additional amount of housing space.
Hence, there is a need for an eloquently simple differential pressure monitoring device.